There are two GNSS which have been fully deployed for a number of years (the US Global Positioning System and the Russian GLONASS) and two more which are under deployment (the Chinese Beidou Navigation Satellite System and the European Galileo system). These systems provide an accurate positioning measurement relying on the same principles: microwave radio frequency (RF) signals are broadcast on the same carrier frequency from a number of satellites which orbit; the signals carry a navigation message which are spread using a PRN (Pseudo Random Noise), the PRN sequence being specific to each transmitter. On the receiver side, the received signals are correlated in tracking loops with locally generated replica of the PRN code, in order to determine the origin of the signals and acquire a tracking position. The processing capabilities of the receivers use the information contained in the navigation message along with the reception time of the message to calculate pseudo-ranges, which are measurements of the distance between the receiver and the satellites. When four or more pseudo-range measurements are calculated from distinct satellites, the receiver can calculate a position, velocity and time (PVT), using a triangulation method or more advanced techniques as for instance the RAIM techniques (Receiver Autonomous Integrity Monitoring).
Among the various phenomena that impact the accuracy the positioning in a GNSS system is the problem of the multipath reflections. Indeed, in GNSS positioning systems, the positioning signals are transmitted via satellites that are usually in line of sight (LOS) with receivers. Thus, at the receiver side, the signal comprises a direct propagation path. However, depending on the propagation environment, it can further comprise paths known as multipaths that results from reflections of the positioning signal over the elements of the environment. These paths are delayed versions of the direct path, which generally come attenuated and phase shifted.
The multipaths create artifacts in the correlation function calculated at the receiver, and consequently affect the pseudo range measurements. Depending on the propagation environment and/or antenna performances, these multipaths can have a power level that is close to, and sometimes higher than, the power level of the direct path. When the tracking loop of the receiver locks on a reflected path instead of the direct path, and as, by definition, the reflected path covers a higher distance than the direct path, it results in a pseudo range measurement error, and consequently in a lower accuracy of the position determined by the receiver.
Tracking a reflected path instead of the direct path can happen frequently, in particular in urban environments, where a high accuracy is required. It is therefore important that GNSS receivers implement techniques to prevent from tracking reflected propagation paths instead of the direct propagation path.
It is known that the polarization of a circularly polarized electromagnetic wave is inverted when the wave is reflected. Thus, to bring robustness against multipaths to the receivers, right hand circularly polarized (RHCP) signals are commonly used. After being reflected, the signal is left hand circularly polarized (LHCP). Using RHCP antennas in the receivers, the power level of LHCP signals, i.e. signals reflected an odd number of times, is greatly decreased. The probability that the receiver tracks a reflected path of the positioning signal is reduced, which brings to the receiver an intrinsic protection against multipath reflections.
Currently, GNSS signals are transmitted using RHCP signals, and most of the receivers embed RHCP antennas, which are sufficient to handle a large number of multipath propagation scenarios. However, the efficiency of this technique highly depends on the quality of the receiving antenna diagram, and some propagation environments, notably in dense urban environments, might still be an issue.
FIG. 1 illustrates a radiation pattern of an antenna in a GNSS receiver as known from prior art. In FIG. 1 are represented the gain of the antenna with respect to the off-boresight angle (OBA), in right hand and left hand circular polarization. Off-boresight angle is expressed with respect to the antenna lighting the zenith direction, an Off-boresight angle of 0° corresponding thus to a vertical up direction of the antenna pattern.
Line 101 represents the gain for a co-polarized electromagnetic wave (in that case, RHCP), namely the gain when the received signal is right hand circularly polarized. It can be observed that the gain is maximal when the signal comes from above the receiver (the off-boresight angle is null). This perfectly suits satellite communications. The gain decreases when the off-boresight angle increases, and is close to zero when the signal comes from the back of the antenna.
Conversely, line 102 represents the gain for the cross-polarized signal, namely the gain when the received electromagnetic wave is LHCP. This gain is rather low when the off-boresight angle is null, and does not reach high values, whatever the off-boresight angle.
In FIG. 1, when the satellite is situated above the receiver (position 103), the gain difference between the right hand and left hand circularly polarized signals is of about 40 dB, which brings a natural protection against reflected propagation paths. This difference decreases conversely to the off-boresight angle. When the off-boresight angle is of about ±90° (meaning that the signal comes almost horizontally, position 104), the difference is of about 10 dB, which is not sufficient to significantly attenuate the reflected propagation paths. When the off-boresight angle is of about ±140° (position 105), this difference is null, and even inverted when the signal comes from a higher angle. There is therefore an issue when the signals come with a high off-boresight angle (this situation generally referred as the signal coming at the rear of the antenna).
In practice, the direct propagation path does not arrive with high off-boresight angles, but reflected paths can do so, for instance when the signal is reflected on the ground. In those cases, the power level of the direct and reflected paths can be equivalent, and the polarization property of the antenna does not play its role of filtering the reflected propagation paths.
It is known, as for instance from MAQSOOD & al., “Effects of Ground Plane on the Performance of Multipath Mitigating Antennas for GNSS”, 2010 Loughborough Antennas & Propagation Conference, to modify the antennas, and particularly their ground plane, to further improve their gain concerning RHCP electromagnetic waves, and to decrease their gain for LHCP electromagnetic waves. Such modifications reduce but do not entirely solve the multipaths problem, and have a cost in terms of complexity, surface and cost of the antenna, which can be an issue for uses in mobile equipments.
Signal processing mitigation algorithms to detect multipath reflections in a positioning signal are also known in the art. Methods to detect multipaths in a GNSS signal comprise methods based on a Maximum Likelihood (ML) estimation of the cross-correlation functions calculated in the tracking loops of the receiver, or the Multipath Estimating Delay-Lock-Loop (MEDLL) method, that aims at estimating the delay and power of all the paths of a signal by studying the shape of the cross-correlation functions. However, these algorithms require a lot of computation power, as many correlators are required, and are sensitive to Gaussian noise. In addition, characterizing the direct path from the reflected path using such algorithms usually requires smoothing consecutive measurements over a period of time, which does not fit with time-varying environments, as for instance for a non-static receiver in a urban environment.
The use of array-antennas is also known for multipath mitigation, but requires many additional antennas and complex signal processing algorithms.
There is therefore a need for a multipath mitigation technique in a GNSS receiver that is robust, and that can be executed in real time, with a low complexity.
US patent application US 2009/0195449 A1 describes a method for determining whether the receiver is in a multipath propagation environment or not. In this patent application, pseudo ranges calculated from signals received on the RHCP antenna are compared with pseudo ranges calculated from signals received on the LHCP antenna. Carrier over Noise ratio measurements (C/NO) are also performed over the signals received on both antennas.
The referenced patent application is based on the following premises:                when the signal only propagates through a direct path, tracking loops associated to the LHCP and RHCP signals are locked at the same position: the pseudo range measurements are substantially equals;        when the signal comprises one or more reflections, the tracking loops dedicated to the signal acquired on the RHCP antenna are locked on the direct path of the signal, while the tracking loops dedicated to the signal acquired on the LHCP antenna are locked on the reflected path of the signal: the pseudo range measurements are different.        
Thus, when a difference performed between the RHCP and LHCP pseudo range measurements exceeds a threshold, it is likely that the receiver is in a multipath propagation environment. This patent application further proposes to use a second criteria, based on a Carrier over Noise ratio measurement, to detect multipaths.
However, the referenced patent application does not solve the problem of detecting the tracking of a reflected path instead of the direct path from the signal acquired on the RHCP antenna. Indeed, when a tracking loop dedicated to the RHCP signal tracks a reflected path of the positioning signal, the difference between the pseudo range measurements acquired from the RHCP and LHCP antenna is almost null. The receiver will then consider that the signal is acquired on the direct path, and put a high level of confidence in a pseudo range measurement which is erroneous.
Actually, a multipath propagation environment will only be detected when the tracking loop processing the signal acquired on the RHCP antenna is locked on the direct path. There is no indication on whether or not the tracking position over the RHCP signal is right or wrong.
The second criteria, based on the C/N measurement, assumes that the antenna gain between RHCP and LHCP signal is constant with respect to the off-boresight angle, which incorrect, as can be observed on FIG. 1. Depending on the bore off sight angle, this gain can go from −40 dB to 40 dB. It is therefore not possible to rely on this criterion, which will raise a high level of false alarms.
As a consequence, the referenced patent application does not solve the problem of distinguishing between the tracking of a direct path and the tracking of a reflected path in a GNSS receiver.